You have the order of importance backwards. Astronomical events were used to help figure out Passover, yes, because they were a reliable way of tracking time. The religious event is what's important—not the position of the moon and stars. You also appear to have adopted the modern idea that the Nativity is more important than Pascha when the opposite was true in early Christianity.
At the time of the Council of Nicaea, there were different groups of Christians who were celebrating the Christian Passover but who disagreed on exactly when to celebrate. Some Christians were celebrating it at the same time as the Jewish Passover. Other Christians determined that it happened on March 25, historically. They were doing it on March 25, whatever day of the week that fell on, sort of the way we celebrate a lot of other feasts. There were also Christians who were doing a sort of variation of what we do now, even if they were doing it on the Sunday nearest the Jewish Passover or the Sunday nearest the 25th. At the Council of Nicaea, this was all settled for the sake of Christian unity.
What was the settlement? As mentioned, there were Christians who were used to celebrating on March 25. What was proposed in the deliberations is the idea of integral age—a supposed Jewish tradition that the date of a prophet’s death would be the same date as his conception. Whether or not there was really such a tradition is a whole different discussion, but the people at the council seemed to think there was. This integral age idea was set forth as part of the settlement. Therefore, if the date that Christ was crucified is March 25 then that would also have been the date of his conception. That was the beginning of the celebration of what we now call the feast of the Annunciation which is celebrated on March 25.
You can read this yourself in
Calculating Christmas by Dr. William Tighe. It talks about integral age and the Annunciation to make this point: If you know the Annunciation is on March 25, what generally happens nine months after a baby is conceived? That's where the date for Christmas comes from. It’s not based on a historical memory of the Church, archeological evidence, appeasing pagans, or anything like that. It's simply the liturgical date when Christ's birth is celebrated.
One last point about this saturnalia nonsense. Even if it had been the case (it's not) that the Church chose to pick something to replace a pagan feast, that’s not syncretism or trying to appease pagans. That's a conquest. We’re taking this time of year and claiming it for Christ. Again, this is not the case for the date of Christmas, but the point is even if it were it wouldn't be syncretism.
Christ's death and resurrection is about the defeat of death, sin, and the demonic powers of paganism. Our job as Christians is to proclaim this victory, the
evangelium, and go retake the world for Christ. The pagan world's months and days were indeed dedicated to demonic powers. The Christian festal calendar is about reclaiming time itself from them and giving them back up to God.
tl;dr There's no excuse to remain ignorant about these things. Stop your foolishness and return to the Christianity of your forefathers.